Abstract
Due to absence of any supporting die, the free surfaces in incremental sheet forming (ISF) experience uneven deformation. This results in rough surfaces, possibly leading to the reduced service life of components. Therefore, it is necessary to analyze and quantify the effects of the application of strain on the free-surface roughness. Moreover, in order to control roughness, both on the free surface and the opposite contact surface, the nature of correlation between the two types of roughnesses needs to be identified by classifying the significance of different process conditions. The present work is a fundamental study to address these points. A series of specimens are produced by subjecting a metallic sheet to a range of ISF strains (13% to 98%). These specimens are then subjected to a number of characterization tests, namely roughness, uniaxial tension, and residual stress tests. The results reveal that the mean free-surface roughness increases non-linearly as the normal strain (stretching + bending) on the free surface increases (where strain state on the surface is as follows: ɛ1 = 0, ɛ2 > 0, γmax = ɛ2 and 1 and 2 are principal directions). The roughness also increases, although linearly, with the post-forming sheet strength, residual stress, and forming force, thereby showing that strain hardening has a direct influence on the roughness in a way that sheet strengthening is achieved at the cost of surface quality. The surface morphology reveals that the free surfaces contained orange peel, slip lines, and micro-voids, with density increasing with strain application, thus indicating the possible influence of tensile stresses on free surface deformation and roughening at an increasing degree with strain. Further analysis of roughness results discloses that the free-surface roughness and the contact-surface roughness are inversely related, because the responses of the two to ISF processing were mutually exclusive. Based on the obtained results, future research directions are also discussed.
Highlights
Incremental sheet forming (ISF) is an advanced sheet forming process economically suitable for producing small-scaled batches for aerospace, biomedical, and automotive parts [1,2,3].The forming tool in this process traverses the peripherally clamped sheet in an incremental fashion to produce a 3D geometry
The meanranged of the from roughness represents the effect of plastic deformation strain on arithmetic the free surface on free-surface roughening in the traditional forming processes [4,5,6,7,8,9]
13% to 83%), thereby endorsing an earlier point that the free-surface roughening in ISF occurred due to surface deformation and defects
Summary
Incremental sheet forming (ISF) is an advanced sheet forming process economically suitable for producing small-scaled batches for aerospace (e.g., noses and cones), biomedical (e.g., customized implants like human leg and skull), and automotive (e.g., car body panels and flanges) parts [1,2,3]. Due to absence of back support, the free surfaces in metal forming endure uneven deformation, resulting in the production of rough components This free-surface roughness promotes strain localization and leads to the premature failure of components during service [4,5]. From the service life and aesthetics viewpoint, it is important to control the roughness on both free and contact surfaces For this purpose, it is necessary to identify how the roughness values on the two surfaces are correlated. Efforts are spent to identify the effects of various process conditions for the two types of roughnesses, with the objective to simultaneously control roughness both on the free and contact surfaces. The presented results, basic in nature, prove beneficial to understanding free-surface roughening and process mechanics and provide a guideline to control two roughnesses simultaneously
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